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Investment Securities (Maturities Table) (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Securities Available for Sale    
One year or less $ 47,740us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue  
Over one year through five years 2,636,819us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue  
Over five years through ten years 777,170us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue  
Over ten years 80,585us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue  
Available-for-sale 3,545,288us-gaap_AvailableForSaleSecurities 3,273,174us-gaap_AvailableForSaleSecurities
Weighted Average Yield, one year or less 3.72%fmer_AvailableForSaleSecuritiesDebtMaturitiesMaturityPeriodOfOneYearOrLessWeightedAverageYield  
Weighted Average Yield, over one year through five years 2.22%fmer_AvailableForSaleSecuritiesDebtMaturitiesMaturityPeriodOverOneYearThroughFiveYearsWeightedAverageYield  
Weighted Average Yield, over five years through ten years 2.92%fmer_AvailableForSaleSecuritiesDebtMaturitiesMaturityPeriodOverFiveYearsThroughTenYearsWeightedAverageYield  
Weighted Average Yield, over ten years 1.94%fmer_AvailableForSaleSecuritiesDebtMaturitiesMaturityPeriodOverTenYearsWeightedAverageYield  
Available-for-sale Securities, Amortized Cost Basis 3,562,537us-gaap_AvailableForSaleSecuritiesAmortizedCost 3,329,117us-gaap_AvailableForSaleSecuritiesAmortizedCost
Weighted-Average Yield 2.38%fmer_AvailableForSaleSecuritiesDebtMaturitiesWeightedAverageYield  
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity 4 years 3 months 1 day  
Securities Held to Maturity    
One year or less 86,024us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue  
Over one year through five years 2,126,878us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue  
Over five years through ten years 475,035us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue  
Over ten years 187,983us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue  
Weighted Average Yield, over one year through five years 1.93%fmer_HeldToMaturitySecuritiesDebtMaturitiesMaturityPeriodOverOneYearThroughFiveYearsWeightedAverageYield  
Weighted Average Yield, over five years through ten years 1.88%fmer_HeldToMaturitySecuritiesDebtMaturitiesMaturityPeriodOverFiveYearsThroughTenYearsWeightedAverageYield  
Weighted Average Yield, over ten years 3.04%fmer_HeldToMaturitySecuritiesDebtMaturitiesMaturityPeriodOverTenYearsWeightedAverageYield  
Weighted Average Yield, Total 5.51%fmer_HeldToMaturitySecuritiesDebtMaturitiesMaturityPeriodWeightedAverageYield  
Held-to-maturity   2,935,688us-gaap_HeldToMaturitySecurities
Held-to-maturity Securities, Fair Value 2,875,920us-gaap_HeldToMaturitySecuritiesFairValue 2,824,240us-gaap_HeldToMaturitySecuritiesFairValue
Held-to-maturity Securities, Amortized Cost 2,903,609us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment 2,935,688us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
Held to Maturity Securities Debt Maturities Weighted Average Yield 2.30%fmer_HeldToMaturitySecuritiesDebtMaturitiesWeightedAverageYield  
Held to Maturity Securities Debt Maturities Weighted Average Maturity 4 years 9 months 4 days  
U.S. Government agencies    
Securities Available for Sale    
One year or less 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Over one year through five years 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Over five years through ten years 2,482us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Over ten years 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Available-for-sale 2,482us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Available-for-sale Securities, Amortized Cost Basis 2,500us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Weighted-Average Yield 1.25%fmer_AvailableForSaleSecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity 3 years 5 months 2 days  
Securities Held to Maturity    
One year or less 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Over one year through five years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Over five years through ten years 24,463us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Over ten years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Held-to-maturity Securities, Fair Value 24,463us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 23,652us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Held-to-maturity Securities, Amortized Cost 25,000us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		 25,000us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Held to Maturity Securities Debt Maturities Weighted Average Yield 1.33%fmer_HeldToMaturitySecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember		  
Held to Maturity Securities Debt Maturities Weighted Average Maturity 4 years 9 months 29 days  
U.S. Treasury obligations    
Securities Available for Sale    
One year or less 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Over one year through five years 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Over five years through ten years 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Over ten years 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Available-for-sale 0us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Available-for-sale Securities, Amortized Cost Basis 0us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Weighted-Average Yield 0.00%fmer_AvailableForSaleSecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity 0 years  
Securities Held to Maturity    
One year or less 5,000us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Over one year through five years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Over five years through ten years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Over ten years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Held-to-maturity Securities, Fair Value 5,000us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		 5,004us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
Held-to-maturity Securities, Amortized Cost 5,000us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		 5,000us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember
Held to Maturity Securities Debt Maturities Weighted Average Yield 0.26%fmer_HeldToMaturitySecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasurySecuritiesMember		  
Held to Maturity Securities Debt Maturities Weighted Average Maturity 0 years 0 months 28 days  
U.S. States and political subdivisions    
Securities Available for Sale    
One year or less 12,365us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Over one year through five years 70,589us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Over five years through ten years 115,140us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Over ten years 29,248us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Available-for-sale 227,342us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 262,367us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Available-for-sale Securities, Amortized Cost Basis 221,052us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 258,787us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Weighted-Average Yield 5.23%fmer_AvailableForSaleSecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity 6 years 0 months 0 days  
Securities Held to Maturity    
One year or less 64,719us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Over one year through five years 78,522us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Over five years through ten years 199,054us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Over ten years 187,983us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Held-to-maturity Securities, Fair Value 530,278us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 475,579us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Held-to-maturity Securities, Amortized Cost 517,824us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		 480,703us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
Held to Maturity Securities Debt Maturities Weighted Average Yield 4.68%fmer_HeldToMaturitySecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember		  
Held to Maturity Securities Debt Maturities Weighted Average Maturity 9 years 4 months 26 days  
Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
One year or less 2,777us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over one year through five years 791,641us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over five years through ten years 176,580us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over ten years 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Available-for-sale 970,998us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 969,922us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Available-for-sale Securities, Amortized Cost Basis 951,839us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 962,687us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Weighted-Average Yield 2.60%fmer_AvailableForSaleSecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity 3 years 9 months 22 days  
Securities Held to Maturity    
One year or less 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over one year through five years 468,348us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over five years through ten years 116,829us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over ten years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Held-to-maturity Securities, Fair Value 585,177us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 559,451us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Held-to-maturity Securities, Amortized Cost 580,727us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 569,960us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Held to Maturity Securities Debt Maturities Weighted Average Yield 2.16%fmer_HeldToMaturitySecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Held to Maturity Securities Debt Maturities Weighted Average Maturity 4 years 3 months 8 days  
Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
One year or less 16,850us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over one year through five years 44,030us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over five years through ten years 42,523us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over ten years 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Available-for-sale 103,403us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 69,567us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Available-for-sale Securities, Amortized Cost Basis 104,176us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 72,048us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Weighted-Average Yield 2.28%fmer_AvailableForSaleSecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Available for Sale Securities, Debt Maturities, Weighted-Average Maturity 4 years 1 month 27 days  
Securities Held to Maturity    
One year or less 16,305us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over one year through five years 22,119us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over five years through ten years 19,671us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over ten years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Held-to-maturity Securities, Fair Value 58,095us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 55,406us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Held-to-maturity Securities, Amortized Cost 58,143us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 56,596us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Held to Maturity Securities Debt Maturities Weighted Average Yield 2.12%fmer_HeldToMaturitySecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Held to Maturity Securities Debt Maturities Weighted Average Maturity 3 years 6 months 5 days  
Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
One year or less 8,248us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialCollateralizedMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over one year through five years 1,579,527us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialCollateralizedMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over five years through ten years 88,792us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialCollateralizedMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Over ten years 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialCollateralizedMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		  
Available-for-sale 1,676,567us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialCollateralizedMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember		 1,518,393us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Securities Held to Maturity    
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Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
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Securities Held to Maturity    
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Over ten years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Collateralized Loan Obligations    
Securities Available for Sale    
One year or less 7,500us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
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Over ten years 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue
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Available-for-sale Securities, Amortized Cost Basis 297,446us-gaap_AvailableForSaleSecuritiesAmortizedCost
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Securities Held to Maturity    
One year or less 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
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Over ten years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Held to Maturity Securities Debt Maturities Weighted Average Maturity 0 years  
Corporate bonds    
Securities Available for Sale    
One year or less 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Over one year through five years 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
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Over five years through ten years 0us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Securities Held to Maturity    
One year or less 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesWithinOneYearFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Over one year through five years 91,099us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Over five years through ten years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Over ten years 0us-gaap_HeldToMaturitySecuritiesDebtMaturitiesAfterTenYearsFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Held-to-maturity Securities, Amortized Cost 90,739us-gaap_HeldToMaturitySecuritiesAmortizedCostBeforeOtherThanTemporaryImpairment
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Held to Maturity Securities Debt Maturities Weighted Average Yield 2.25%fmer_HeldToMaturitySecuritiesDebtMaturitiesWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Held to Maturity Securities Debt Maturities Weighted Average Maturity 3 years 0 months 10 days  
Total debt securities    
Securities Available for Sale    
Available-for-sale 3,542,314us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Available-for-sale Securities, Amortized Cost Basis 3,559,563us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Fair Value, Inputs, Level 2 [Member]    
Securities Available for Sale    
Available-for-sale 3,203,127us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member		 2,922,527us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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Securities Held to Maturity    
Held-to-maturity Securities, Fair Value   2,824,240us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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Fair Value, Inputs, Level 2 [Member] | U.S. States and political subdivisions    
Securities Available for Sale    
Available-for-sale   262,367us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Fair Value, Inputs, Level 2 [Member] | Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
Available-for-sale   969,922us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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Fair Value, Inputs, Level 2 [Member] | Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
Available-for-sale   69,567us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Fair Value, Inputs, Level 2 [Member] | Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
Available-for-sale   1,518,393us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialCollateralizedMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Fair Value, Inputs, Level 2 [Member] | Residential Collateralized Mortgage-Backed Securities Issued by Non-US Government Agency    
Securities Available for Sale    
Available-for-sale 1us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialCollateralizedMortgageBackedSecuritiesIssuedByNonUsGovernmentAgencyMember		 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_ResidentialCollateralizedMortgageBackedSecuritiesIssuedByNonUsGovernmentAgencyMember
Fair Value, Inputs, Level 2 [Member] | Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
Available-for-sale   102,268us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= fmer_CommercialCollateralizedMortgageBackedSecuritiesIssuedByUsGovernmentSponsoredEnterprisesMember
Fair Value, Inputs, Level 2 [Member] | Collateralized Loan Obligations    
Securities Available for Sale    
Available-for-sale 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember
Fair Value, Inputs, Level 2 [Member] | Corporate bonds    
Securities Available for Sale    
Available-for-sale 0us-gaap_AvailableForSaleSecurities
/ us-gaap_FairValueByFairValueHierarchyLevelAxis
= us-gaap_FairValueInputsLevel2Member
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
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/ us-gaap_FairValueByFairValueHierarchyLevelAxis
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/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Fair Value, Inputs, Level 3 [Member]    
Securities Available for Sale    
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Securities Held to Maturity    
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Fair Value, Inputs, Level 3 [Member] | U.S. States and political subdivisions    
Securities Available for Sale    
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Fair Value, Inputs, Level 3 [Member] | Residential Mortgage Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
Available-for-sale 0us-gaap_AvailableForSaleSecurities
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Fair Value, Inputs, Level 3 [Member] | Commercial Mortgage Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
Available-for-sale 0us-gaap_AvailableForSaleSecurities
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Fair Value, Inputs, Level 3 [Member] | Residential Collateralized Mortgage-Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
Available-for-sale 0us-gaap_AvailableForSaleSecurities
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Fair Value, Inputs, Level 3 [Member] | Residential Collateralized Mortgage-Backed Securities Issued by Non-US Government Agency    
Securities Available for Sale    
Available-for-sale 6us-gaap_AvailableForSaleSecurities
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Fair Value, Inputs, Level 3 [Member] | Commercial Collateralized Mortgage Backed Securities Issued by US Government Sponsored Enterprises    
Securities Available for Sale    
Available-for-sale 0us-gaap_AvailableForSaleSecurities
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Fair Value, Inputs, Level 3 [Member] | Collateralized Loan Obligations    
Securities Available for Sale    
Available-for-sale   293,687us-gaap_AvailableForSaleSecurities
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Securities Held to Maturity    
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Fair Value, Inputs, Level 3 [Member] | Corporate bonds    
Securities Available for Sale    
Available-for-sale   $ 50,644us-gaap_AvailableForSaleSecurities
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